This is a multicenter, randomized, double-blind, placebo-controlled, parallel-group, 4-week study involving 360 patients who are in acute relapse with a diagnosis of schizophrenia or schizoaffective disorder, and who have a previous history of responding to neuroleptics. Aripiprazole, a novel anti-dopaminergic agent, is a potent D2 receptor antagonist. In phase-II studies, aripiprazole was shown to be effective in treating both positive and negative symptoms of psychosis with minimum drug-related side effects, and the most consistent results were obtained with a 30 mg/day dose, which also demonstrated a unique early onset of efficacy. This phase-III study is undertaken to further examine the safety and efficacy of aripiprazole in a larger number of patients. Since approximately 30% of schizophrenic patients do not respond to currently available treatments, a group of patients treated with haloperidol will be included in this study to document response to a D2 antagonist in the patient population enrolled in this study. Upon inclusion, the patients will be submitted to a five-day placebo washout from prior therapy and then randomized to one of the following 4 double-blind treatment arms for 4 weeks: 1) 15 mg oral aripiprazole, once daily after breakfast; 2) 30 mg oral aripiprazole, once daily after breakfast; 3) 10 mg oral haloperidol, once daily after breakfast; and 4) oral placebo, once daily after breakfast.